Which method involves using chemicals to change the structure of hair?

The method that involves using chemicals to change the structure of hair is chemical relaxing. This process is designed to alter the natural wave pattern of the hair by breaking down the disulfide bonds in the hair's cortex through the application of specific chemicals. The result is straighter hair, making it particularly beneficial for individuals with curly or wavy hair who desire a smoother appearance.

Chemical relaxing is distinct from other methods. Thermal styling uses heat to achieve styling effects and does not change the hair structure permanently. Mechanical curling involves the use of tools such as rollers or curling irons to temporarily alter the shape of the hair without altering its chemical structure. Hair cutting simply involves trimming the hair and does not affect its internal structure or texture. As a result, chemical relaxing is the only method among the options provided that fundamentally changes the hair's structural makeup due to the chemical processes involved.